ABPI is expressed as the ratio of the ankle blood pressure to the highest brachial (upper arm) blood pressure. Generally, systolic blood pressure values are measured to determine this index. When a certain portion of an artery is affected by occlusive disease, blood pressure downstream of the occlusion in the direction of flow is lower than upstream of the occlusion so that an abnormal ankle and brachial blood pressure index is obtained and arterial disease can be diagnosed.
A known technique to measure ABPI involves the patient being supine and having rested for 5 to 10 minutes. The systolic pressures are then measured sequentially in each of the four limbs by applying a cuff to the upper arms and at the ankles. Usually a hand held Doppler probe is used to listen to the blood flow either in the brachial artery for blood pressure in the arms or in the dorsalis pedis and posterior tibial arteries for ankle blood pressures. However, the technique has difficulties in identifying the arteries in the foot using the Doppler probe as well as maintaining the blood vessel contact with Doppler probe during inflation and deflation of cuff. Also, the blood pressure is continually changing and pressures measured at the beginning of the test may be disassociated with pressures taken at the end of the test.
The use of arterial photo-plethysmography (PPG) can overcome the disadvantages of Doppler. However, it is dependent on the presence of fingers or toes and adequate arterial flow which may be compromised either by temperature or arterial disease. More recently, pulse oximeters have been used for the detection of arterial pulses when taking systolic pressures. However, their use is likely to introduce errors in the pressure readings during deflation of the barometric cuff due to the inherent averaging process within the instrument.